1. Technical Field
The present disclosure relates to a radar apparatus.
2. Description of the Related Art
In recent years, a study has been conducted on high-resolution radars using microwaves or millimeter waves. Also, in order to improve outdoor safety, there are demands for development of wide-angle radars that can sense not only vehicles but also pedestrians.
Wide-angle pulse radars for sensing vehicles and pedestrians receive signals in which reflected waves from short-distance targets (e.g., vehicles) and reflected waves from long-distance targets (e.g., people) are mixed. Thus, the wide-angle pulse radars require radar transmitters that transmit pulse waves or modulated pulse waves having a low range sidelobe characteristic. The wide-angle pulse radars further require radar receivers having a wide reception dynamic range.
For wide-angle pulse radars of related art, pulse compression radars have been proposed that use Barker codes, M-sequence codes, or complementary codes as pulse waves or modulated pulse waves. A complementary-code generating method is disclosed in Budisin, S. Z. “New complementary pairs of sequences”, Electronics Letters, 1990, 26, (13), pp. 881-883.
Complementary codes are generated, for example, in the following manner. That is, complementary codes having a code length L=4, 8, 16, 32, . . . , 2P are sequentially generated based on a code sequence a=[1 1] and a code sequence b=[1 −1] including elements 1 or −1 and having complementarity. With wide-angle pulse radars using general pulse codes, the required reception dynamic range increases as the code length increases, but with wide-angle pulse radars using complementary codes, the peak sidelobe ratio (PSR) can be reduced with a smaller code length.
Thus, with such known wide-angle pulse radars, even when a reflected wave from a target (or an object) at a short distance and a reflected wave from a target (or an object) at a long distance are mixed, it is possible to reduce the dynamic range required for the reception. However, when the known wide-angle pulse radars use M-sequence codes, the PSR becomes 20 log (1/L), and thus codes having a code length L (e.g., L=1024 for PSR=60 dB) that is larger than that of complementary codes are needed in order to obtain low range sidelobes.
For wide-angle pulse radars of related art, there have been proposed codes (hereinafter referred to as “Spano codes”) obtained by contriving the arrangement order of complementary codes in a segment in which a reflected wave from a target is received during transmission of pulse codes (hereinafter referred to as a “code transmission segment”) (see Spano, E. and O. Ghebrebrhan “Sequences of complementary codes for the optimum decoding of truncated ranges and high sidelobe suppression factors for ST/MST radar systems”, IEEE Transactions on Geoscience and Remote Sensing Vol. 34, pp. 330-345, 1996). Also, a radar apparatus using Spano codes is disclosed in Japanese Unexamined Patent Application Publication No. 2002-214331.
However, with the known radar apparatus of the related art, when isolation between a transmitting antenna and a receiving antenna is insufficient, signal components of transmission signals which leak to the receiving antenna (i.e., transmission leakage signals) increase in a time segment (a code transmission segment) in which pulse codes are transmitted from the transmitting antenna. In addition, when a target having a large RCS (Radar Cross Section) (a large-RCS object, typified by a vehicle) exists at a distance close to the radar apparatus of the related art, the signal components reflected from the large RCS object increase.
In those cases, in the radar apparatus of the related art, when the amplitude of reception baseband signals output from a reception radio-frequency (RF) circuit exceeds the dynamic range of an analog-to-digital (A/D) converter, nonlinear distortion due to clipping occurs. Also, in the radar apparatus of the related art, when an input to a low-noise amplifier (LNA) in the reception RF circuit reaches an input level in a saturation region, nonlinear distortion occurs.
When nonlinear distortion components are generated in the radar apparatus of the related art, the range sidelobes increase owing to influences of the nonlinear distortion components in a time segment corresponding to a pulse code width of a reflected wave or transmission leakage signals from a large RCS object at a timing at which the reflected wave or the transmission leakage signals arrive(s). Thus, the radar apparatus of the related art has a possibility that the performance of detecting a reflected wave from a low-RCS object deteriorates in that time segment.